1. Field of the Invention
The present invention relates to a filter comprising a combination of thin film resonators, and more particularly to a thin film resonator and a method for making the same that are used in the filter.
2. Description of the Related Art
Generally, a filter used for a communication device includes a combination of thin film resonators.
FIG. 1 is a cross-sectional view of a conventional thin film resonator. Referring to FIG. 1, a conventional thin film resonator 100 includes a piezoelectric layer 130, an upper electrode 140 and a lower electrode 120 provided respectively on the upper and lower surfaces of the piezoelectric layer 130, and a membrane layer 110 (also called a “dielectric layer”) vibratably supporting the lower electrode 120 on a substrate 10.
The resonant frequency, one of resonant characteristics of the thin film resonator 100, varies depending on the thickness t1 of the piezoelectric layer 130. In recent small-sized thin film resonators, a new technology is adopted to adjust the resonant frequency using the so-called mass loading effect that causes a change of mass according to the thickness t2 of the upper electrode 140, without changing the thickness t1 of the piezoelectric layer 130 which results in a greater variation of the resonant frequency.
In the conventional thin film resonator 100, the lower electrode 120 receives chemical and mechanical impacts during repeated deposition and etching processes during manufacturing of the thin film resonator 100. To endure such chemical and mechanical impacts, the lower electrode 120 should be made of a material having high chemical and mechanical strength, such as molybdenum (Mo) or tungsten (W), rather than a light metal, such as aluminum. However, both molybdenum and tungsten have a greater vibration transfer loss than aluminum due to their high damping factor and are not easily processed. These materials increase the manufacturing cost and complicate the manufacturing process, while lowering overall efficiency of the thin film resonator 100.
As described above, recently it has become possible to adjust the resonant frequency of the thin film resonator 100 by changing the thickness t2 of the upper electrode 140. The change of the thickness of the upper electrode 140 leads to a much smaller variation of the resonant frequency, as compared to the thickness change of the piezoelectric layer 130. However, as shown in FIG. 2, the resonant frequency of the thin film resonator 100 greatly varies in a wide range of hundreds of MHz even when the thickness of the upper electrode 140 slightly changes by 0.1 μm. Accordingly, it is difficult to manufacture a precise filter that requires a resonant frequency difference between a series resonator group 310 and a parallel resonator group 320 (see FIG. 8) to be tens of MHz.